412 Prof. M. Schultze on Polytrema miniaceum. 



There were evidently three possibilities to be taken into con- 

 sideration here : — 



1. Polytrema might be a Sponge with a reticulated calcareous 

 skeleton, forming a network like the horny substance of the 

 officinal sponge. Within the gaps of this network would be the 

 organic sponge-substance which forms siliceous spicules. 



2. Polytrema might be a Polythalamian. The organic sub- 

 stance within the calcareous skeleton would then be a Rhizopod- 

 body, and the siliceous spicules must have penetrated accident- 

 ally, or been eaten, or derived from a Sponge living parasitically 

 in the Polythalamian. 



3. The structure might, like Carpenteria in the opinion of 

 Gray and Carpenter, represent a transitional form between 

 Sponges and Polythalamia, inasmuch as the calcareous walls 

 possess Foraminiferous structure, but the body of the animal is 

 allied to the Sponges in its faculty of producing siliceous 

 spicules. 



As regards the first possibility, the preparation of thin sections 

 of the calcareous mass shows that it does not consist of calcareous 

 rods anastomosing in a netlike form like the horny skeleton of 

 the common Sponge, but of lamellse which enclose a system of 

 anastomosing chambers nearly similar in form and size, and, 

 further, that these lamellse (the walls of the chambers, as has 

 already been stated) possess an exquisite Foraminiferous struc- 

 ture. A thin section of Polytrema, perpendicular to the surface 

 and viewed by transmitted light, is shown in PI. VII. fig. 3. 

 The colour of the calcareous walls is reddish, even in thin sec- 

 tions. They are all penetrated by the ordinary pore canals of 

 the Polythalamia, which usually run perpendicularly and by the 

 shortest course towards the surface. The thickness of the cal- 

 careous walls varies, although no definite rule could be detected. 

 In fig. 3 a thicker calcareous wall runs from a towards b on the 

 surface of the section of Polytrema ; but similar thick walls are 

 also frequently met with through considerable spaces in the in- 

 terior of the structure. In very thin sections a stratified struc- 

 ture may be detected, especially in the thicker walls; and, corre- 

 sponding with this, the canals of the wall exhibit a peculiar 

 division into segments]*, which may be perceived very distinctly, 

 after the solution of the calcareous matter, in the membranous 

 tubes which occupied each canal (fig. 9). 



The pore canals of the surface are very closely approximated, 

 being at an average distance of 0*009 mill, from each other. In 

 the interior septa, on the contrary, they are often much further 

 apart. The width of the canals themselves is 0*004-0"006 mill., 



* Figured in the same way by Carpenter in Carpenteria, Phil. Trans. 

 1860, pi. 22. fig. 15. 



